Apparatus for bonding metallic panels

ABSTRACT

The apparatus of the invention is particularly directed to achieving proper electrical bonding current distribution between the various components of the panel in such a manner as to achieve a suitable temporary bond prior to the subjection of the panel to the heat bonding step. The apparatus includes electrical bonding means consisting of a pair of longitudinally translatable electrodes cooperative with an electrode bar. The longitudinally translatable electrodes are mounted for movement in engagement with the exterior surface of one of the face sheets of the panel, and the elongated electrode bar is inserted into the interior of the panel in supportive relationship with a core element being bonded into the core structure of the panel and to the inner surface of the respective face sheet. The longitudinally translatable pair of electrodes is so spaced as to cause them to deliver electrical bonding current to spaced portions of the panel and core structure having different electrical characteristics. Furthermore, the longitudinally translatable pair of electrodes are connected in a circuit with the elongated electrode in such a manner that maximum current equalization at both of the electrodes is achieved with minimum current bypassing through the surface sheet. In addition, the elongated electrode bar is provided with means adapted to cooperate with longitudinally translatable electrodes to subject different portions of the panel and core structure to different current densities to accommodate for different thicknesses of the face sheet and core structure being bonded to one another.

Unite States Campbell atent [54] APPARATUS FOR BONDING METALLIC PANELS[72] Inventor: James R. Campbell, South Laguna,

Calif.

[73] Assignee: Thomas P. Mahoney,

Monica, Calif. a part interest April 19, 1971 Santa [22] Filed:

21 Appl. No.: 135,013

Related US. Application Data [52] US. Cl. ..219/82, 219/117 HD [51] Int.Cl. ..B23k 11/06 [58] Field of.Search.....219/l 17, 117 HD, 82, 83, 84,

Primary Examiner-J. V. Truhe Assistant Examiner-B. A. ReynoldsAttorney-Mahoney, Hombaker & Schick [57 v w ABSTRACT The apparatus ofthe invention is particularly directed 5] Sept. 5, 1972 to achievingproper electrical bonding current distribution between the variouscomponents of the panel in such a manner as to achieve a suitabletemporary bond prior to the subjection of the panel to the heat bondingstep. The apparatus includes electrical bonding means consisting of apair of longitudinally translatableelectrodes cooperative with anelectrode bar. The longitudinally translatable electrodes are mountedfor movement in engagement with the exterior surface of one of the facesheets of the panel, and the elongated electrode bar is inserted intothe interior of the panel in supportive relationship with a core elementbeing bonded into the core structure of the panel and to the-innersurface of the respective face sheet.

The longitudinally translatable pair of electrodes is so spaced as tocause them to deliver electrical bonding current to spaced portions ofthe panel and core structure having different electricalcharacteristics. Furthermore, the longitudinally translatable pair ofelectrodes are connected in a circuit with the elongated electrode insuch a manner that maximum current equalization at both of theelectrodes is achieved with minimum current bypassing through thesurface sheet. In addition, the elongated electrode bar is provided withmeans adapted to cooperate with longitudinally translatable electrodesto subject difierent portions of the panel and core structure todifferent current densities to accommodate for different thicknesses ofthe face sheet and core structure being bondedto one another.

6 Claims, 6 Drawing Figures APPARATUS FOR BONDING METALLIC PANELSCROSS-REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTIONRecent advances in aeronautical and aerospace technology have caused ademand for structural materials which would meet the stringentrequirements of supersonic exposure, elevated temperatures and thevarious other physical phenomena incidental to supersonic speeds such ashigh energy sound levels. Among materials which have been offered tocope with the problems generated in the aeronautical and astronauticalfields have been various types of composite panels usually consisting ofa core structure supporting first and second face sheets in spacedrelationship with each other.

Initially, the, face sheets were fabricated from aluminum and the corestructure fabricated from such relatively perishable materials as resinimpregnated paper secured to the interior surfaces of the aluminum facesheets by various types of natural or synthetic adhesives. Therequirements of the advancing technologies left ultimately to theutilization of aluminum core structures secured to aluminum face sheetsby the same resinous adhesives as previously utilized with the fibrouscores. In time, a brazing process involving the utilization of brazingmaterials as a means of attaching the aluminum face sheets to thealuminum core structures was developed.

Subsequently, stainless steel face sheets and core structures weresubstituted for the aluminum face sheets and core structures.Ultimately, an all-welded stainless steel honeycomb core panel wasdeveloped, as disclosed in my U.S. Pat. No. 2,910,153, issued Oct. 27,1959, entitled Structural Panel of Honeycomb Type. Apparatus and methodswere also developed to accomplish the automatic welding of the stainlesssteel core structures to the face sheets utilized in conjunctiontherewith, as exemplified by my previously issued U.S. Pat. Nos.2,931,883, issued Mar. 29, 1960 entitled Method and Apparatus forFabricating Structural Panel and Core Therefor. and 3,015,715, issuedJan. 2, 1962 entitled Method and Apparatus for Resistance Welding.

Because of the fact that the stainless steel sheet materials utilized inthe fabrication of the panels in accordance with my invention weresubstantially foil-like in nature, that is, having a thickness in therange extending between two-thousandths and five-thousandths of an inch,considerable difficulty was encountered in developing the apparatus andmethods disclosed in my previously mentioned patents. However, with theadvent of the product and the apparatus, numerous applications of thestainless steel welded honeycomb core panel were made in critical areasof aircraft and aircraft engines and in space technology. However,recent requirements in astronautics and aeronautics have dictated theuse of even more sophisticated constructions where weight, fatigueresistance and temperature factors are encountered that require theutilization of titanium or stainless steel panel of even greaterstructural integrity.

Considerable developmental and research work has been engaged in tofabricate welded panels characterized by the incorporation of titaniumfoil cores having titanium face sheets affixed thereto by the weldingprocesses of the above referenced applications, and such panels haveexhibited physical characteristics superior in some aspects to those ofthe previously mentioned stainless steel panels. However, in supersonicapplications or in applications where elevated temperatures, stringentand large fatigue requirement loads are imposed upon the panels, theconventional weldments connecting the face sheets to the core structuresof the panels tend to create stress risers which constitute areas ofload concentration susceptible to incipient failure resulting inpossible destruction of the ;panel by causing delamination of the facesheets from operative relationship with the associated core structure,and also possible fatigue of the face sheets.

Consequently, I have developed a method and apparatus for creating ametallic panel structure of the general character of the metallic panelspreviously utilized, but substantially devoid of the stress risercharacteristics encountered when conventional welding techniques areutilized. Although the method and apparatus will be describedhereinbelow as applied specifically to the fabrication of panelsincorporating titanium cores having titanium face sheets affixedthereto, it will, of course, be obvious to those skilled in the art thatthe teachings and practices of the invention may be applied with equalcogency to materials other than titanium, and it is not intended thatsaid teachings and practices be limited to the specific materialsdisclosed.

OBJECTS AND SUMMARY OF THE INVENTION The teachings of my inventionembrace two general areas, that is, a method of fabricating metallicpanels and apparatus for performing one of the critical steps of themethod. As a result of the practice of the steps of the method and theutilization of the apparatus disclosed hereinbelow, large metallicpanels fabricated from titanium and substantially devoid of the stressrisers incident to the utilization of the conventional welding processhave been fabricated and have been tested with very satisfactoryresults. The titanium panels are particularly desirable in variousaircraft structures where exposure to significant loads at moderatelyelevated temperatures is encountered, such as in the external wingsurfaces of supersonic aircraft. The substantial absence of stressrisers eliminates the possibility of incipient failure of the panelsresulting from tension fatigue of the face sheets or the delamination ofthe face sheets from operative relationship with the associated coreand, further, the intimacy of the bond between the face sheets and coreresulting from the practice of the method and in the utilization of theapparatus of my invention is due to molecular diffusion between theadjacent surfaces of the core structure and the face sheets of thepanel.

It is, therefore, an object of my invention to provide a method offabricating a composite metallic panel which is characterized,primarily, by the practice of two major steps, that is, an initialelectric bonding step and a subsequent heat bonding step.

The electrical bonding step can be accomplished, as will be adverted toin greater detail hereinbelow, by the application of carefullycontrolled electrical bonding current and pressure to cause initialbonding between the interior surfaces of the face sheets and thecontiguous areas of the core structure to occur. The consequent bondresulting from the electrical bonding step is characterized by the factthat it is sufficient to maintain the components of the panel inoperative relationship with one another, but is essentially insufficientto resist design loads and environments to which the panel may beultimately subjected.

After the core and face sheets of the panel have been electricallybonded to one another, the panel is subjected to a heat bonding stepwhich structurally integrates the components of the panel to such anextent as to prevent the delamination of the face sheets of the panelfrom operative relationship with the core structure absent themechanical disruption and damage of the core structure and face sheets.The heat bonding step may be practiced by the utilization of well-knowndiffusion bonding techniques or may be accomplished by exposure of theelectrically bonded panel to inductive current in a reducing atmosphere.

Attempts have been made in the past to fabricate metallic panelsincluding metallic core structures and face sheets by diffusion bondingprocesses alone, but such attempts have been characterized by theinability to produce commercial quantities of panel because of thedifficulties encountered in obtaining the requisite intimacy of contactbetween the contiguous surfaces of the core structure and face sheets.That is, the pressures required to achieve such contiguous intimacy haveresulted in the ultimate collapse or distortion of areas of the panelbeing diffusion bonded if the panel was of commercial size. However, bythe practice of the sequential steps of preliminary electrical bondingand subsequent diffusion bonding, the elimination of the need forexternal pressure during the diffusion bonding step has resulted in theachievement of panels characterized by co-planarity of the surfaces ofthe face sheets and the undiminished structural integrity of the facesheets and core structure interposed therebetween.

A further object of my invention is the provision of a method of theaforementioned character wherein the electrical bonding step takes placeduring the application of high pressure to the exterior surface orsurfaces of the face sheets while the core structure disposedintermediate the face sheets is supported to resist the collapse of thecore structure during the application of the high pressures to saidexterior surfaces of said face sheets. In addition, the relatively lowbonding current utilized eliminates the oxidation encountered whenwelding current densities are utilized and the consequent contaminationof the areas immediately adjacent the bond.

Of course, oxidation of the metal adjacent the bond is inimical to thesuccessful heat bonding of the components of the panel by the diffusionprocess since the contamination of the panel in the oxidized areasprevents the proper molecular interchange and diffusion between thecontiguous surfaces of the panel components and also negates thesalutary effect of the previous cleansing step to which the componentshave been subjected.

Another object of my invention is the provision of an apparatus for usein electrically bonding the components of a sheet metal panel inoperative relationship with one another which includes electricalbonding means constituted by spaced, longitudinally translatableelectrodes adapted to be imposed upon the exterior surface of arespective face sheet and an associated elongated electrode bar adaptedto be disposed in supportive relationship with an underlying corestructure adjacent said face sheet, said electrode bar having meansthereupon for causing contiguous areas of said face sheet and corestructure to be exposed to different bonding current densities wheresuch contiguous areas are of different physical thicknesses.

Another object of my invention is the provision of an apparatus of theaforementioned character wherein the spaced electrodes are connected insuch a manner as to prevent the bypassing of bonding current through theface sheets to maintain a maximum current density balance between saidspaced electrodes.

A further object of my invention is the provision of an apparatus of theaforementioned character wherein the spacing between said longitudinallytranslatable electrodes is such that one electrode is superimposed uponan area of said panel characterized by greater thickness while the otherof said electrodes is disposed in overlying relationship with an area ofsaid panel characterized by lesser thickness than the other area.

Other objects and advantages of my invention will be apparent from thefollowing specification and the accompanying drawing which is for thepurpose of illustration only and in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary top plan viewshowing a pair of longitudinally translatable electrodes of theapparatus of the invention engaged in performing the electrical bondingstep of the method of the invention;

FIG. 2 is a fragmentary, vertical sectional view taken from the brokenline 22 of FIG. 1;

FIG. 3 is an enlarged fragmentary view taken from the broken line 3-3 ofFIG. 1;

FIG. 4 is an enlarged, fragmentary side elevational view of a portion ofthe electrode bar utilized in the electrical bonding apparatus of theinvention;

FIG. 5 is a schematic circuit diagram showing the balancing circuitryutilized in conjunction with the electrical bonding means of theinvention; and

FIG. 6 is an enlarged, vertical fragmentary sectional view showing themanner in which the nodal areas of the core structure of a panel to besubjected to the method steps of my invention are juxtaposed to eachother during the electrical bonding step.

DESCRIPTION OF EMBODIMENTS OF THE METHOD AND apparatus OF THE INVENTIONReferring to the drawing and particularly to FIGS. 1-3 thereof, I show ametallic panel 10 adapted to be fabricated by the method and apparatusof the invention. The panel 10 includes upper and lower face sheets 12and 14 and a honeycomb core structure 16, said honeycomb core structurebeing constituted by a plurality of core strips 18 which are ofgenerally corrugated configuration, as best shown in FIG. 1 of thedrawing, and which have internested nodal. areas or nodes with the nodes20 of the core strips 18 which project upwardly being depressed, as at22, FIG. 3, to provide for the facile interfitting of the upwardlyprojecting nodes 20 of a core strip 18 within the downwardly projectingnodes 20 of a previously located core strip, as best shown in FIGS. 1and 3 of the drawings, and as more particularly described in mypreviously referred to US. Pat. No. 2,910,153.

Although the method of my invention is described as being applied in thepresent application to the specific panel disclosed herein and shown inthe various Figures of the drawing, it, of course, will be obviousto'those skilled in the art that various types of composite metal panelshaving metallic cores and metallic face sheets may be fabricated by thesteps of the process and the use of the apparatus of my inventionwithout departing materially in any respect from the teachings thereof.

For instance, the core strips 18 of the core 16 of the panel 10incorporate laterally directed flanges 24 which are disposed incoplanarity with the inner surfaces of the face sheets 12 and 14 andwhich provide linear areas of substantial width sufficient to permit theperformance of the electrical bonding and the heat bonding steps. Thisstructure is in sharp contrast with conventional stainless steel brazedpanel structures wherein the stainless steel core is characterized bythe fact that the opposite sides of the core have no flanges thereuponand constitute, in effect, sharp edges which prevent the application ofthe electrical bonding and heat bonding steps because there isinsufficient area to permit the operation of the steps and theconsequent structural affixation of the core to the inner surfaces ofthe face sheets.

As previously mentioned, the metallic components of the panel 10 arefabricated from titanium in dimensions which are characteristic ofmetallic foils. For instance, titanium foils of 0.003 inches inthickness have been used in the core and foils of 0.040 inches inthickness have been used in the face sheets. For instance, I have foundthat when the panel 10 is fabricated from heat treatable alpha-betaalloys, the combinationof 6Al-4V alloy face sheets and 3Al-2.5V corestrips provides a panel 10 having very high mechanical propertiessuitable for application in demanding aeronautical and astronauticalenvironments.

However, while the method and apparatus of the invention is described asapplied in conjunction with titanium alloy foils, it is not intended tolimit the teachings of the invention to any particular material, sinceother suitable materials, including stainless steel, are available towhich said teachings'may be applied.

As previously indicated, one of the basic requirements for theachievement of structural integrity of the panel 10 is a high degree ofcleanliness of the components, that is, the face sheets 12 and 14 andthe flanged core strips 18, prior to the practice of the electricalbonding step. To achieve such cleanliness of the components, pickling orother cleansing steps are essential. In addition, the time between thepickling operation and the fabrication of the panel 10 should be reducedto the minimum. In practice I have found that a maximum exposure of thetreated components to atmosphere is 24 hours. If manufacturingprocedures indicate a greater exposure, the component parts should bestored in a nonoxidizing environment until they are to be utilized.

Other cleanliness controls entail the utilization of coolants which havebeen suitably treated to eliminate the maximum of contaminants therefromso that detritus from said coolants will not be deposited upon thesurfaces of the components of the panel 10 during the electrical bondingstep which entails the utilization of various liquid coolants such aswater. As a matter of fact, the utilization of deionized water for floodcooling during the electrical bonding process is desirable. In addition,the apparatus for accomplishing the electrical bonding step should belocated in dust-free environment to eliminate the possibility ofcontamination of the panel 10 during the electrical bonding step.

Furthermore, care must be taken that burrs and chips on the componentsresulting from cutting operations thereupon be eliminated from saidcomponents, and that contamination by the electrical bonding apparatusitself (lubricants, dust or metal particles) be obviated to the greatestextent possible.

After the proper cleansing precautions have been taken, the componentface sheets 12 and 14 and core strips 18 are deposited, in a manner tobe described in greater detail below, in the apparatus of the invention.Since the general structural relationship of the components of theapparatus isdisclosed in my US. Pat. No. 3,077,532, issued Feb. 12,1963, entitled, Method and Apparatus for Fabricating Honeycomb Core. adetailed description thereof is not deemed necessary since the apparatusof the present invention is limited primarily to the control of theelectrical bonding current in the performance of the electrical bondingstep.

In preparation for the electrical bonding step, the first and secondface sheets 12 and 14 may be oriented in either a vertical or horizontalposition in spaced relationship with each other, as best shown in FIG. 3of the drawing. The spacing of the face sheets 12 and 14 is determinedby the height of the core strips 18 to be inserted into the spacebetween said face sheets.

The electrical bonding apparatus includes electrical bonding meansconstituted by a pair of longitudinally translatable electrical bondingrollers 26, said electrical bonding rollers being translatable in alongitudinal path and being maintained in a predetermined spatial andoperative relationship with each other by means of a yoke 28.

Operatively associated with the electrical bonding means is an elongatedelectrode 30 which is adapted to be inserted in the space between theface sheets 12 and 14 carrying a single core strip 18 for operativeengagement with the previously inserted core strips, said elongatedelectrode bar 30 incorporating a plurality of bonding projections 32, asbest shown in FIGS. 3 and 4 of the drawing, the bonding projections 34engageable with the nodal areas of the core strips 18 being smaller andof greater number than the larger and fewer projections 36 adapted toengage the portions of the core strips 18 intermediate the nodal areas.

Consequently, the bonding current density is greater where three layersof titanium material are encountered at the overlapped, internestednodal areas 20 and reduced current densities are experienced where theintermediate portions of the flanges 24 constitute two layers ofmaterial with the adjacent face sheets 12 and 14. i

For instance, the relatively small bonding projections 34 can beprovided in a dimension of approximately 0.025 "x 0.025 inches and thelarger projections 36 may have a dimension of approximately 0.060 X0.045 inches. Of course, the dimensions of the bonding projections canbe altered to meet different bonding requirements but the basicprinciple of providing more numerous and smaller projections at thethicker areas of the panel always applies.

In addition to the control of the current densities achieved by thedifferential sizing of the bonding projections 34 and 36, the elongatedelectrode 30 also serves as a supporting means for the core structureand the relevant core strips 18 during the electrical bonding process.This physical support by the electrode during the electrical bondingprocess is important since the bonding rollers 26 are subjected topressures of an order of 20,000 to 40,000 psi to insure the intimatecontact between the adjacent surfaces of the flanges 24 of the corestrips 18 and the interior contiguous surfaces of the face sheets 12 and14. Of course, the flanges 24 provide linear areas of substantial crosssection to facilitate the performance of the electrical bonding step,said flanges being disposed in horizontal planes and substantialparallelism with the inner surfaces of the face sheets 12 and 14. Theimportance of the control of pressure on the face sheets 12 and 14 andassociated core 18 is attributable to the fact that relatively lowelectrical bonding current is utilized to avoid the creation of spotwelding phenomena which would be deleterious during the heat bondingstep.

Undesirable welding phenomena to be avoided dur- 0 ing electricalbonding include excessive thermally induced stresses resulting from thecreation of spot welds and thermally and mechanically induceddeformations in the face sheets. Such deformations are generallyconstituted by a displacement of metal occuring during the plastic ormolten stage of the weld pulse, and result in stress risers which cannullify desired designed criteria if created in the panel. Furthermore,as previously pointed out, one of the criteria of the success of,

the heat bonding process is maximum cleanliness and oxidation ordissociation phenomena characteristic of spot welding would result inundesirable contamination at the critical interface between thehorizontally oriented flanges 24 of the core 16 and the inner surfacesof the face sheets 12 and 14.

Consequently, the achievement of consistent and precise bonding currentcontrol sufficient to bond the adjacent surfaces of the core structure16 to the inner surfaces of the face sheets 12 and 14 and the relevantadjacent internested and overlapped faces of the core strips 18 to oneanother, but insufficient to oxidize or contaminate the faces, is abasic prerequisite for the ultimate success of the heat bonding step.

During the electrical bonding step, after the elongated electrode 30 hasbeen inserted into the space between the face sheets 12 and 14, carryinga core strip 18, and has located said core strip 18 with the respectiveflanges 24 at the nodal areas 20 and overlapped and in internestedrelationship, the pairs of bonding rollers 26 are translatedlongitudinally across the respective surfaces of the face sheets 12 and14 in a linear path coincident with the linear, horizontally orientedsurfaces provided by the flanges 24 of the underlying internested corestrips 18. Because of the fact that two bonding rollers 26 are utilizedin each of the electrical bonding means, and because there is adifference in the thickness of the materials to be bonded due to thefact that the combination of the face sheets and internested nodal areasprovides three layers of materials while only two layers of materialsare provided between the internested areas, it is desirable to have acurrent density increase available for the bonding roller 26 engagedwith the interengaged overlapping flange area underlying the respectiveface sheet.

To accomplish this desired result, the bonding rollers 26 are so spacedthat, as best shown in FIG. 3 of the drawing, a bonding roller 26 ofeach pair will overlie a triple layer portion of the panel 10 beingbonded while the other roller will overlie a double layer portion of thepanel being bonded.

Consequently, the current flow always occurs through the double layer ofmaterial at the same time that it occurs through the triple layer and,thus, the relatively high resistance path A to G is always subjected tothe same current flow as the relatively low resistance path B to Hthrough the elongated electrode bar 30. This automatic adjustment of thebonding current density insures that the increased current density willbe achieved at the triple layer areas as distinguished from the doublelayer areas having smaller current density requirements and also insuresthat a proper electrical bond will be achieved at these areas withoutincurring the danger of subjecting the panel to a welding action whichwould result in the undesirable oxidation phenomena referred to indetail hereinabove.

It will be noted that the showing of the spacing of the bondingrollers,26 in FIG. 3 of the drawing illustrates the use of aconventional transformer hook-up wherein a single transformer 40 isutilized with each pair of welding rollers 26. However, I havediscovered that more effective bonding current control may be achievedby improved circuitry connected to the bonding rollers 26, as bestillustrated schematically in FIG. 5 of the drawing. With the improvedcircuit 42, the firing control 44 is connected to a pair of weldingtransformers 46 and 48 whose primaries 52 and 54, respectively, areconnected in series and whose secondaries 56 and 58 are respectivelyconnected to an associated welding roller and the electrode bar 30.

As the result of the utilization of the circuit 42, current flow in thesecondaries 56 and 58 tends to be substantially identical. For instance,if one secondary is I closed and the other secondary is open, verylittle current will flow in the one secondary. Within limits, thecircuit is a constant current circuit when fired with a constant voltagesupply.

One of the significant advantages of the use of the circuit 42 is thatsubstantially the same potential would be impressed on the assembledcomponents of the panel 10 at each bonding roller so that no by-passedcurrent down the face sheets 12 and 14 would be encountered as suchby-passed current is encountered with the circuitry as shown in FIG. 3of the drawing. The circuit 42 will thus provide balanced power on bothrollers 26, permit widely divergent control settings for differentthickness of face sheets, and compensate for or eliminate variable IRdrop effects due to electrode bar resistance variation with roller'travel. The circuitry shown in FIG. 3 involves no variable IR dropeffects. A single bonding roller connected to a welding transformersecondary leg, the other leg of which is connected to the bar, will workso as to control current density if the firing control is so programmed.However, it has the disadvantage of requiring twice the weld time andconsequent cost increase, and it would also be necessary to program thefiring control for variable IR drop in the bar.

By utilizing the apparatus described hereinabove or one or more aspectsof the apparatus, it is possible to achieve and electrically bondedpanel which will meet the criteria previouslyadverted to for the heatbonding step.

The nodal areas 20 of the individual core strips 18 are, ofcourse,provided with and incorporate adjacent vertical areas 62 whichare juxtaposed to each other, as best shown schematically in FIG. 6 ofthe drawing, during the process of insertion of the nodal areas of onecore strip into internested relationship with a previously inserted corestrip 18. In the welded configurations of panel of the same basicstructure as the panel 10, it has previously been found necessary toweld the vertical areas 62 of the nodal portions 20 of the core strips18 in operative relationship with one another. However, it has beenascertained that, due to the forming process of the core strips 18, thevertical areas 62 incorporate bulges or bows which cause portions of thevertical areas 62 to automatically engage each other, and it has alsobeen found that it is not necessary to bond these vertical areas 62 toeach other during the electrical bonding step, although, of course, suchelec trical bonding of the vertical areas 62 of the nodal areas orportions 20 of the core strips 18 can be accomplished. Even thoughunbonded, I have ascertained that the intimate contact of the verticalareas 62 of the nodal portions 20 of the core strips 18 causes them tobe bonded to each other during the heat bonding step of the method of myinvention. I

Of course, one of the major problems encountered in the proper bondingof the components of the-panel in operative relationship prior to theheat bonding step is the control of the bonding temperatures which, ofcourse, are contingent upon such otherfactors as the thickness of theface sheets; the pressure of the bonding rollers; the roller speed; theconfiguration, thickness and material of the core; electrical currentpulse characteristics; roller material and configuration; the force withwhich the core strips are internested with each other and the linevoltage.

In the over-all apparatus, as disclosed in the above referenced patents,the criterion of the heat produced by the bonding current is acombination of the transformer tap and the phase shift including powerfactor correction and cool time as termined by the adjustment of heat ortemperature control knobs.

On many occasions attempts have been made to empirically determine thetemperature created at the interface between adjacent facing surfacesduring spot welding of various sheet metals to each other but theseattempts have failed because the insertion of any temperature detectingmeans between the elements to be welded does not provide an indicationas to where the specific reading took place. Consequently, I have reliedupon empirical experimentation to determine the setting for the specificmaterials utilized in the panel 10, in the present instance, thetitanium alloy mentioned hereinabove.

For instance, at the beginning or the end of an experimental panel Ihave allowed for 12 extra core strips and have tabulated the core stripsin order to keep a record thereof. I have then reduced the currentsetting from a predetermined norm by increasing the phase shift apredetermined amount approximately equal to 0.2 volts peak at therollers. I have then inserted two more core strips and again reducedheat'by the same means and in the same percentages as set forthhereinabove. The process has been repeated until all six pairs of corestrips have been subjected to the heat reduction.

' The quality of the bond for each zone of two core strips subjected toprogressively reduced temperatures has then been inspected to establishthe minimum bonding current.

In addition, there are certain visual criteria which can be utilized toinspect the bond in order to determine whether a satisfactory bond willoccur during the heat bonding step. One of the primary criteria utilizedis to inspect the bond to determine that there is an absence of spotwelding phenomena such as discoloration of the material, theme orstructural deformation of the material at the bonding area or notableindication of oxidization of the material at the contiguous surfaces ofthe face sheets and the core. If such indications are present thebonding process has failed because of the fact that the oxidized areasas indicated by deformation, discoloration, or other welding phenomenawill result in oxygen contamination during the heat bonding process andprevent the creation of a satisfactory molecular interchange between thesurfaces of the face sheets and the core.

A positive visual indication of a satisfactory electrical bond is aslightly frosty appearance at the interfaces between the face sheets andthe opposite sides of the core secured to said face sheets with notableabsence of yellow or bluish discolorations adjacent the bonded zones.

The panel 10 is completed by the repeated insertion of core strips 18into operative relationship with one another until the size of paneldesired has been attained by the use of the electrical bonding step insecuring the core strips 18 into a core structure 16 and in securing theresulting core structure 16 to the inner surface of face sheets 12 and14. The panel 10 is then ready to be subjected to the heat bonding step,which in the present practice of the invention, is accomplished byheating in a vacuum.

To accomplish the heat or diffusion bonding of the panel, I haveutilized an automatic vacuum furnace which will accept a large sizedpanel or panels. Titanium getter shields of CF. titanium may be placedabout the panels in the furnace to protect the panel from excessive heatradiation and gas contamination. The he ating rates are controlled byautomatic control equipment and the vacuum is maintained at 10 TORR orgreater, if possible. The temperature to which the panel 10 is exposedmay be in the range of l,600 Fahrenheit for approximately 2 hours tocause the molecular diffusion of the metal at the interfaces between thecore 16 ad the face sheets 12 and 14. Time and temperature can be variedindependently or reciprocally over a wide range to satisfy specificrequirements. Certain stainless steels would be advantageously diffusionbonded at 2,300- F. The utilization of dry hydrogen atmospherefacilitates the diffusion bonding of stainless steel. In any event, ageneral rule of thumb for establishing the heat bond temperature is thatit should be above the solution annealing temperature and below thetemperature at which grain growth is excessive.

Because of the fact that the interfaces of the core and face sheets aremaintained in intimate juxtaposition with each other due to the previouselectrical bonding step, and because there is a substantially completeabsence of contaminants at said interfaces by either vagrant or inducedcontaminants, the maximum diffusion at the said interfaces occurs. As amatter of fact, the success of the diffusion bonding of the interfacesis a criterion of the success of the electrical bonding step since anycontamination, whether vagrant or induced, in the electrical bondingstep will result in an impaired diffusion bond between the interfaces ofthe components of the panel.

The panels may be suspended in the diffusion bonding oven or may beplaced upon flat platens if it is so desired. In addition, it has beenfound that the panels will assume the configuration of contoured diesupon which they are placed during the heat bonding process as carriedout in the diffusion bonding oven.

During the diffusion bonding process the rate of heating is carefullycontrolled while maintaining the 10 TORR vacuum. Since major outgassingmay be encountered between l,000 and 1,300 F., the heating rate untilthis range is reached can be as low as 10 F. minute. When the l,300 F.temperature is reached, it may be temporarily arrested to stabilize thevacuum.

The heating rate from 1,300 F. to the diffusion bonding temperature maybe as low as 5 F. minute. The relatively slow heating rates minimizecontamination due to possible heavy outgassing.

Under certain conditions, it may be desirable to heat rapidly to atemperature where grain growth would tend to start, then drop back to atemperature slightly above the solution anneal temperature for the heatbonding soak. With titanium GAL-4V, this could involve rapid heating to1,725 F., then a reduction to 1,500" F., immediately upon reaching 1,725F for the heat bond soak time of perhaps 1 or more hours.

I claim:

1. In an electrical bonding apparatus for use in bonding an electricallyconductive core to electrically conductive face sheets where said corehas interengaged portions providing a greater thickness with said facesheets than other portions of said core, the combination of: asupporting electrode disposed in operative relationship with a portionof said core being electrically bonded; and movable electrical bondingmeans cooperative with said electrode and engageable with an adjacentexternal surface of a relevant face sheet, said electrical bonding meansbeing translatable with reference to said face sheet and incorporatingspaced electrodes, the spacing between said electrodes being such thatwhen one electrode is disposed over the interengaged portions of saidcore, the other of said electrodes is disposed in operative relationshipwith a zone of said core intermediate said interengaged portions.

electrode.

2. In an electrical bonding apparatus for use in bonding an electricallyconductive core to electrically conductive face sheets where said corehas interengaged portions providing a greater thickness with said facesheets than other portions of said core, the combination of: asupporting electrode disposed in operative relationship with a portionof said core being electrically bonded; and movable electrical bondingmeans cooperative with said electrode and engageable with an adjacentexternal surface of a relevant face sheet, said electrical bonding meansbeing translatable with reference to said face sheet and an electricalbonding power supply controllable to provide a greater bonding potentialwhen the movable electric bonding means are disposed over theinterengaged portions of said core.

3. In an electrical bonding apparatus for use in bonding an electricallyconductive core to electrically conductive face sheets where said corehas interengaged portions providing a greater thickness with said facesheets than other portions of said core, the combination of: anelectrode disposed in operative relationship with the portion of saidcore being bonded, said electrode having means thereupon whereby greatercurrent density is impressed upon the aforesaid interengaged portions ofsaid core structure and whereby lesser current density is imposed uponportions of said core structure intermediate said interengaged portions;and electrical bonding means cooperative with said electrode andtranslatable in alignment therewith to cause the electrical bonding ofsaid core structure to the inner surfaces of said face sheets.

4. The apparatus of claim 3 in which said means for imparting greatercurrent density to said interengaged portions of said core structure areconstituted by relatively smaller welding projections on said electrodeand whereby said means for causing lesser current density areconstituted by relatively larger projections on said 5. In an electricalbonding apparatus, for electrically bonding the core and face sheets ofa metallic panel in operative relationship with one another, thecombination of: an elongated electrode adapted to be inserted betweensaid face sheets and in juxtaposition to a portion of said core to bebonded to the inner surfaces of said face sheets; and electrical bondingmeans consisting of spaced electrodes engageable with a correspondingsurface of a respective face sheet and adapted to be translated linearlyin correspondence with said electrode; and a bonding current sourceincluding a pair of transformers, each having its primary connected inseries with the other transformer and its secondary connected at oneterminal to a respective electrode and at its other terminal to saidelongated electrode.

6. The apparatus of claim 5 in which at least two spaced electrodes areprovided and said bonding current source includes at least twotransformers having their primaries connected in series with each otherand said secondaries connected to a respective spaced electrode and tosaid elongated electrode, the connections of said transformers being sopolarized that an increment of power from said bonding current sourceproduces a voltage of like polarity at each of said spaced electrodesand a voltage of opposite polarity at said elongated electrode.

1. In an electrical bonding apparatus for use in bonding an electricallyconductive core to electrically conductive face sheets where said corehas interengaged portions providing a greater thickness with said facesheets than other portions of said core, the combination of: asupporting electrode disposed in operative relationship with a portionof said core being electrically bonded; and movable electrical bondingmeans cooperative with said electrode and engageable with an adjacentexternal surface of a relevant face sheet, said electrical bonding meansbeing translatable with reference to said face sheet and incorporatingspaced electrodes, the spacing between said electrodes being such thatwhen one electrode is disposed over the interengaged portions of saidcore, the other of said electrodes is disposed in operative relationshipwith a zone of said core intermediate said interengaged portions.
 2. Inan electrical bonding apparatus for use in bonding an electricallyconductive core to electrically conductive face sheets where said corehas interengaged portions providing a greater thickness with said facesheets than other portions of said core, the combination of: asupporting electrode disposed in operative relationship with a portionof said core being electrically bonded; and moVable electrical bondingmeans cooperative with said electrode and engageable with an adjacentexternal surface of a relevant face sheet, said electrical bonding meansbeing translatable with reference to said face sheet and an electricalbonding power supply controllable to provide a greater bonding potentialwhen the movable electric bonding means are disposed over theinterengaged portions of said core.
 3. In an electrical bondingapparatus for use in bonding an electrically conductive core toelectrically conductive face sheets where said core has interengagedportions providing a greater thickness with said face sheets than otherportions of said core, the combination of: an electrode disposed inoperative relationship with the portion of said core being bonded, saidelectrode having means thereupon whereby greater current density isimpressed upon the aforesaid interengaged portions of said corestructure and whereby lesser current density is imposed upon portions ofsaid core structure intermediate said interengaged portions; andelectrical bonding means cooperative with said electrode andtranslatable in alignment therewith to cause the electrical bonding ofsaid core structure to the inner surfaces of said face sheets.
 4. Theapparatus of claim 3 in which said means for imparting greater currentdensity to said interengaged portions of said core structure areconstituted by relatively smaller welding projections on said electrodeand whereby said means for causing lesser current density areconstituted by relatively larger projections on said electrode.
 5. In anelectrical bonding apparatus, for electrically bonding the core and facesheets of a metallic panel in operative relationship with one another,the combination of: an elongated electrode adapted to be insertedbetween said face sheets and in juxtaposition to a portion of said coreto be bonded to the inner surfaces of said face sheets; and electricalbonding means consisting of spaced electrodes engageable with acorresponding surface of a respective face sheet and adapted to betranslated linearly in correspondence with said electrode; and a bondingcurrent source including a pair of transformers, each having its primaryconnected in series with the other transformer and its secondaryconnected at one terminal to a respective electrode and at its otherterminal to said elongated electrode.
 6. The apparatus of claim 5 inwhich at least two spaced electrodes are provided and said bondingcurrent source includes at least two transformers having their primariesconnected in series with each other and said secondaries connected to arespective spaced electrode and to said elongated electrode, theconnections of said transformers being so polarized that an increment ofpower from said bonding current source produces a voltage of likepolarity at each of said spaced electrodes and a voltage of oppositepolarity at said elongated electrode.